1. Field of the Invention
This invention relates to catalysts for the preparation of vinyl esters and more particularly it relates to an improved palladium-gold alloy catalyst.
2. Description of the Prior Art
U.S. Pat. No. 3,190,912 discloses the preparation of catalyst for preparing unsaturated organic esters wherein the catalyst consists of unsupported or supported metals of the platinum or palladium group or oxides or salts thereof, either organic or inorganic. Supported catalyst is prepared by dissolving the metal salt or salts in, for example, water, adding the support and evaporating the solvent. The noble metal is distributed uniformly throughout the support. The activity of the catalyst can be promoted by the addition of a metal halide promoter.
U.S. Pat. No. 3,275,680 discloses a palladium catalyst on aluminum oxide support which when promoted with alkali acetate has been found suitable for the production of organic acetates, especially vinyl acetate. The palladium catalyst is deposited throughout the catalyst support. There is no specific disclosure in the patent of combinations of two noble metals or of the use of alloys. The space-time-yield is approximately 50 grams of vinyl acetate per hour per liter of catalyst at about 150.degree. C. This corresponds to a specific activity of about 1.8 grams of vinyl acetate per gram palladium per hour (4% palladium on Al.sub.1 O.sub.3 support of 700 grams per liter density). The catalyst is prepared by impregnating the support with an aqueous solution of the noble metal salt and precipitating the noble metal on the support by reduction.
U.S. Pat. No. 3,743,607 discloses a catalyst containing palladium, gold and an alkali metal acetate. Reproducing the examples of this patent it can be determined that the palladium and gold are distributed throughout the catalyst support. The catalyst produced approximately 50 to 150 grams vinyl acetate per hour per liter of catalyst at 170.degree. C. Based upon 1.1% palladium and gold on an about 600 grams per liter density support this corresponds to a specific activity of about 7.6 to 22.7 grams of vinyl acetate per gram precious metal per hour (at 170.degree. C). The catalyst is prepared by impregnating a carrier such as silicic acid, aluminum oxide, or silicate, aluminum phosphate, etc. with an aqueous solution of a palladium salt and of a gold salt and evaporating the resulting mixture to dryness. The palladium and gold salts are then reduced to their metallic state by a reducing agent. The catalyst is then washed with water, impregnated with a solution of sodium acetate and upon drying is ready for use.
British Pat. No. 1,333,449 describes a palladium-gold catalyst suitable for the preparation of vinyl acetate. It can be established that the process of this patent results in the palladium salts being deposited throughout the catalyst support. The catalyst is prepared by impregnating the catalyst support with a solution of palladium acetate, barium aceto-aurate and potassium acetate in acetic acid and subsequently drying it. The catalyst is disclosed to be a palladium salt and other additives, such as gold, gold salts, alkaline earth metal salts and alkaline metal salts supported on a carrier. The space-time-yield is about 240 grams of vinyl acetate per hour per liter of catalyst at a temperature of 180.degree. C., which calculates to about 31.5 grams of vinyl acetate per gram precious metal per hour.
U.S. Pat. No. 3,775,342 discloses a vinyl acetate catalyst. Reproducing the examples of this patent it can be determined that an interior band of palladium-gold alloy is deposited on the catalyst support. The catalyst yielded from about 235 grams vinyl acetate per liter of catalyst per hour at 140.degree. C. up to about 452 grams of vinyl acetate per hour per liter of catalyst at 176.degree. C. This corresponds to a specific activity of 49 and 94 grams of vinyl acetate per gram precious metal per hour at 140.degree. C. and 176.degree. C., respectively. The catalyst is disclosed to be prepared by treating the catalyst support simultaneously or successively, with or without intermediate drying, with a solution of palladium and gold salts and a solution which contains compounds that are able to react on the catalyst support with the palladium-gold salts to form water insoluble palladium and gold compounds and then transforming the water insoluble palladium and gold compounds into the noble metals by treatment with reducing agents, and removing the water soluble compounds by washing. It is recommended to employ an alkali metal carboxylate, e.g., alkali metal acetate, on the catalyst such that after drying the catalyst contains from 1 to 30 percent by weight alkali metal carboxylate. In all of the examples, in which the catalyst support was treated successively with the solution of palladium and gold compounds and the precipitating solution the catalyst was dried between these successive treatments.
Published Japanese Patent Application No. 48-10135/1973 describes a process for preparing surface impregnated supported vinyl acetate catalyst. In a preliminary step a small amount of reduced metal (such as gold) is deposited throughout the porous support. This is followed by impregnation of the required amount of palladium catalyst which then is deposited in a surface layer around the preformed metal particles. The palladium catalyst is deposited in a surface layer which has a thickness of about 15% or less of the particle radius. The use of alkali metal acetate salts as a cocatalyst is recommended. This catalyst is disclosed to yield about 170 grams of vinyl acetate per liter of catalyst per hour at 120.degree. C. and up to about 295 grams of vinyl acetate per hour per liter of catalyst at 130.degree. C. These correspond to specific activities of 11 and 19 grams of vinyl acetate per gram of precious metal per hour at 120.degree. C. and 130.degree. C., respectively.
British Patent Specification 1,283,737 discloses a method of preparing a supported metal catalyst by contacting the support with a solution of a compound of a metal such as platinum, palladium, etc. and converting the deposited compound to the metallic state, the porous catalyst support having been impregnated with an alkaline solution and saturated with from 25 to 90% of water or an alcohol. The degree of penetration of metal into the catalyst support pellet is disclosed to be up to 50% of the pellet radius. The use of a minor amount of an activator, such as sodium and potassium acetate, is recommended. Catalysts prepared in this fashion were disclosed to yield up to 18 grams of vinyl acetate per hour per gram of palladium at 140.degree. C. Data contained in the examples indicates that for penetration levels of up to 37 and 25%, respectively, the amount of vinyl acetate produced in grams per hour per gram of palladium increases with increasing penetration.
Published Patent Application No. 73713/74 discloses a process for the preparation of catalyst suitable for the production of vinyl acetate by employing palladium and gold deposited on a porous catalyst support consisting essentially of silica or alumina, which support has a particle radius of 1.0 and 5.0 millimeters and a pore volume of 0.1 to 2.0 ml./g. At least 90% of each of the palladium and gold is disclosed to be present in a surface layer which extends less than 30% of the particle radius from the surface and wherein the percentage of gold is 5 to 60% by weight of the sum of palladium and gold present. The catalyst contains 0.3 to 3.0% of palladium and 0.0157 to 1.8% of gold, both based on the weight of the support. Such catalyst are disclosed to possess improved ability to produce vinyl acetate. Supported catalyst having the above improved properties is disclosed to be prepared by preliminary depositing a very small amount of a reduced metal, such as palladium and/or gold, on the porous carrier, for example, by impregnating the carrier with a salt solution of the metal to be deposited, drying the catalyst support and reducing the metallic salt to metal. This is followed by depositing the necessary amount of palladium and gold to be employed in the catalyst which will then adhere to the surface layer of the catalyst support and is stated to improve catalytic efficiency as compared to conventional catalysts. Production of vinyl acetate at 110.degree. C. was disclosed to be as high as 158 grams per hour per liter of catalyst and at 120.degree. C. as high as 192 grams of vinyl acetate per hour per liter of catalyt. These correspond to specific activities of 6 grams of vinyl acetate per gram of precious metal per hour at 110.degree. C. and 7.4 grams of vinyl acetate per gram of precious metal per hour at 120.degree. C.
U.S. Pat. No. 3,939,199 discloses the use of a catalyst wherein palladium is deposited on a catalyst support which has a pore volume of from 0.4 to 1.2 ml./g. and wherein less than 10% of the total pore volume is attributable to micropores having a diameter of less than 30 Angstrom. The usual inert substances such as silicic acid, silicates, aluminum silicates, titanium oxide, zirconium oxide, and various glasses are disclosed as the suitable support materials. For the preparation of vinyl acetate from ethylene and oxygen, the catalyst support is impregnated with a solution of palladium acetate, cadmium acetate and potassium acetate in acetic acid with subsequent drying. The catalyst yielded 305 grams of vinyl acetate per hour per liter of catalyst at 180.degree. C. which corresponds to a specific activity of 60 grams of vinyl acetate per gram of palladium per hour at 180.degree. C.